1. Field of the Invention
This invention relates to ac inputs for digital processors and more particularly to such an input which rejects false signals created by stray capacitance, and which also provides high voltage transient withstand.
2. Background Information
Typically, digital processors utilize analog-to-digital converters to convert ac signals to digital, logic level signals for input to, and use by, the processor where the magnitude of the ac signal is of interest. In some applications, certain ac input signals are applied directly to the processor, such as where the ac signal is used as a logic input and its absolute magnitude is not of interest. The input circuits within a digital processor used in these applications, have clamping diodes which clip the half cycles of the ac waveform to the logic level used by the processor, for instance about 5 volts. For 120 volt ac signals, this produces an input signal that basically looks like a square wave. However, similar large ac signals on adjacent leads can be capacitively coupled to the lead connected to the ac input resulting in false inputs to the digital processor. The capacitive coupling is typically a result of poor wiring techniques, but cannot always be eliminated.
One application in which such direct input of ac signals to a digital processor is used is an electrical contactor or starter where 120 volt ac control signals are applied to ac input of a microprocessor in a digital control for the device. These control signals can be generated by "start" and "stop" push buttons located a distance from the processor necessitating long leads which are subject to capacitive coupling. The false signals can be derived from signals of higher voltage than the control voltage and even of different frequency from the true signal. It is common to use the phase angle and the magnitude of the signal on the input to detect the coupling of false inputs. However, as the phase and the voltage or frequency of the coupled signal vary, holes in the capacitive detection develop, and nuisance signals are accepted. It is not possible to insure that the end user will follow accepted wiring practices which would minimize such false signals.
Therefore, there is a need for an improved ac input for a digital processor with reduced susceptibility to capacitively coupled false signals.
There is a further need for such an ac input with a high voltage transient withstand capability.
There is a need for such an improved ac input for digital processors which does not require phase measurement.
There is also a need for such an improved ac input for digital processors which is economical and easily implemented.